Conductive structures

ABSTRACT

A conductive structure is used in electric variable resistance devices to provide changes in electrical resistance with movement and changes in pressure, the variable resistance device comprising externally connectable electrodes ( 10 ) bridged by an element ( 14 ) containing polymer and particles of metal, alloy or reduced metal oxide, said element ( 14 ) having a first level of conductance when quiescent and being convertible to a second level of conductance by change of stress applied by stretching or compression or electric field, the device further comprising by means ( 18 ) to stress the element ( 14 ) over a cross-sectional area proportional to the level of conductance required.

TECHNICAL FIELD

This invention relates to conductive structures used in electricvariable resistance devices to provide changes in electrical resistancewith movement and changes in pressure. The structures can also provideelectrical isolation and shielding and allow a start resistance to beset. Further, they can provide a leakage path for electrostaticvoltages, add a degree of movement and tactility to operation and inpreferred forms can respond to the presence of chemical, biological orradioactive species.

BACKGROUND ART

Reference is made to prior applications: A: PCT/GB98/00206, published asWO 98/33193; and B: PCT/GB99/00205, published as WO 99/38173, whichdisclose polymer compositions having the electrical property ofinsulation when quiescent but conductance when stressed mechanically orin electric fields. Typically, in a high resistance state (typically10¹² ohm. cm), they change to a low resistance state (typicallymilliohm. cm) by the application of such stress. It appears that theeffective resistance of the polymer component phase is reduced owing toelectron-tunnelling and carrier trapping. When in such a state, thepolymer composition is able to carry high electric current densities,even though there are no complete metallic pathways, i.e. thecomposition is below the percolation threshold. The disclosure of theseprior applications is incorporated herein by reference and extractstherefrom are quoted hereinbelow. The invention may use materialsdescribed therein but is not limited thereto.

SUMMARY OF THE INVENTION

According to the invention in its first aspect an electric variableresistor comprises externally connectable electrodes bridged by anelement containing polymer and particles of metal, alloy or reducedmetal oxide, said element having a first level of conductance whenquiescent and being convertible to a second level of conductance bychange of stress applied by stretching or compression or electric field,characterised by comprising means to stress the element over across-sectional area proportional to the level of conductance required.

In this specification:

the term ‘variable resistor’ may include a switch, because the range ofresistance available may amount to open circuit; and

the particles of metal, alloy and reduced metal oxide, whetherencapsulated by polymer or not, and whether stressed or stressable toconductance, will be referred to as ‘strongly conductive’;

The stressing means may comprise an actuator having variable geometry atthe site of application, for example an oblique shoe or a selectivelyactivatable array of pins or radiation beam sources. A variable resistorpreferred for simplicity comprises the element and, matching the crosssection thereof, a layer composed of insulating or weakly conductivematerial and containing interstices accessible to mobile fluid. (Mobilefluid need not in fact be present, e.g. the variable resistor may beoperated in a vacuum). More particularly the element may be of ayielding consistency permitting penetration through the layer to anextent depending on an applied compression force. Preferably the elementcomprises a material that itself increases conductance when compressed.

The layer has a base structure selected suitably from foam, net, gauze,mat or cloth and combinations of two or more of these. The basestructure and the material from which it is made affects, and may bechosen to suit, the physical and mechanical limits and performance ofthe overall structure and also for a moderating influence on the amountof creep normally associated with flexible conductive polymers.

Particularly useful layers comprise one or more of open-cell polymerfoam, woven or non-woven textile e.g. felt, possibly with fibre/fibreadhesion, and 3-dimensional aggregations of fibre or strip.

The element may have a base structure of the same general type as thelayer, but chosen to suit its particular function in the variableresistor. For example an element of collapsed structure may be used incombination with a non-collapsed layer, as described further below.Preferably the element base structure contains interstices accessible tomobile fluid.

The invention also provides, as a new article, a porous body having abase structure of polymer containing interstices accessible to mobilefluid and containing polymer and particles of metal, alloy or reducedmetal oxide, said body having a first level of electrical conductancewhen quiescent and being convertible to a second level of conductance bychange of stress applied by stretching or compression or electric field,characterised in that the base structure is a collapsed foam or cloth.Such a porous body may have at least one of the preferred features setout herein in relation to the variable resistor.

In the variable resistor the stressing means may be effective to forexample: (a) apply conductance-increasing stress and/or (b) reverse suchstress or act against pre-existing stress.

If the stressing means acts by compression or stretching, it may befor_example mechanical, magnetic, piezo-electric, pneumatic and/orhydraulic. Such application of stress can be direct or by remotecontrol. If compressive, it may expel mobile fluid from the intersticesof the element and/or layer. In a simple switch the fluid is air and theelement and/or layer will be open to atmosphere. Whether mobile fluid ispresent or not, the element and/or layer may be resilient enough torecover fully alone or aided by a resilient operating member such as aspring. For reversing mechanical stress the element and layer may be setup in a closed system including means to force the mobile fluid into theinterstices. Such a system may provide a means of detecting movement ofa workpiece acting on the fluid outside the variable resistor.

The mobile fluid may be elastic, for example a non-reactive gas such asair, nitrogen or noble gas or possibly a readily condensable gas.Alternatively the fluid may be inelastic, for example water, aqueoussolution, polar organic liquid such as alcohol or ether, non-polarorganic liquid such as hydrocarbon, or liquid polymer such as siliconeoil. In an important case the fluid is a test specimen to which theconductance of the variable resistor is sensitive.

Among the materials suitable for making the element and layer are:

for net, gauze, mat or cloth:

hydrophobic polymers such as polyethylene, polyalkyleneterephthalate,polypropylene, polytetrafluoroethylene, polyacrylonitrile, highlyesterified and/or etherified cellulose, silicone, nylons; and

hydrophilic polymers such as cellulose (natural or regenerated, possiblylightly esterified or etherified), wool and silk;

for foam:

polyether, polystyrene, polypropylene, polyurethane (preferably havingsome plasticity), silicone, natural or synthetic rubber.

Whichever material is used for the element, it is preferably availablein a form having relatively large interstices (e.g. 50-500 microns) andcapable of collapse by compression by a factor of 2 to 8 leaving furthercompressibility.

Typically the element has 2 dimensions substantially greater than thethird. Thus it is of a sheet-like configuration, for example thethickness 0.1 to 5, especially 0.5 to 2.0 mm. Its other dimensions arechosen to suit convenience in manufacture and user requirements, forexample to permit contacting with a test specimen in a sensor accordingto the third aspect of the invention. If the element is to be stressedelectrically, its cross-sectional area should be subdivided intoelectrically separate sub-regions, to permit the required partialactivation. Preferably the element is anisotropic, that is, compressibleperpendicularly to its plane but resistant to compression or stretchingin its plane.

The content of strongly conductive material in the element is typically500-5000 mg/cm³. The size of the variable resistor can be chosen from anextremely wide range. It could be as small as a few granules ofencapsulated metal; it could be part of a human movement area. In auseful example, since it can be made of flexible material, it mayincorporated into a garment.

If the layer is to be weakly conductive, this may be due to containing‘semi’ conductive materials, including carbon and organic polymers suchas, polyaniline, polyacetylene and polypyrrole. The invention can beused to change the physical and electrical properties of theseconductive materials. The weak conductance of the layer may,alternatively or additionally be due to a strong conductor, typically aspresent in the element, but at a lower content, for example 0.1 to 10%of the level in the element.

The element may contain weakly (‘semi’) conductive material as listedabove. If the element has interstices, these may contain such a weakconductor, for example open-cell foam pre-loaded during manufacture witha semi-conductive filler to give a start resistance to a switch orvariable resistor or to prevent the build up of static electricity on orwithin such a device.

The element and the layer, that is, the conductive and non-conductivestrata, can be manufactured separately and placed over each other orheld together using an adhesive—see FIG. 2c below. In an alternative—seeFIG. 2b below—the layer may be integral with the element, theconcentration of the strongly conductive material being graded. Thus anexample of element and layer is a thin foam sheet which if stressed iscapable of strong electrical conductance on one side whilst the oppositeside remains electrically insulating or weakly conductive. The sheet canbe produced by loading the interstices of a non-conductive open-cellfoam sheet part of the way through its thickness with a stronglyconductive powder or granule. This produces a conductive stratum of foamoverlying a non-conductive stratum of foam. The conductive material canbe kept in place within the foam sheet by an adhesive or bycross-linking the foam after loading.

In the variable resistor the strongly conductive material may be presentin one or more of the following states:

a constituent of the base structure of the element;

(b) particles trapped in interstices and/or adhering to surfacesaccessible to the mobile fluid;

(c) a surface phase formed by interaction of strongly conductive fillerparticles (i or ii below) with the base structure of the element or acoating thereon.

Whichever state the conductive material is present in, it may beintroduced:

(i) ‘naked’; that is, without pre-coat but possibly carrying on itssurface the residue of a surface phase in equilibrium with its storageatmosphere or formed during incorporation with the element. This isclearly practicable for states (a) and (c), but possibly leads to a lessphysically stable element in state (b);

(ii) lightly coated, that is, carrying a thin coating of a passivatingor water-displacing material or the residue of such coating formedduring incorporation with the element. This is similar to (i) but mayafford better controllability in manufacture;

(iii) polymer-coated but conductive when quiescent. This is exemplifiedby granular nickel/polymer compositions of so high nickel content thatthe physical properties of the polymer are weakly if at all discernible.As an example, for nickel starting particles of bulk density 0.85 to0.95 this corresponds to a nickel/silicone volume ratio (tappedbulk:voidless solid) typically over about 100. Material of form (iii)can be applied in aqueous suspension. The polymer may or may not be anelastomer. Form (iii) also affords better controllability in manufacturethan (i);

(iv) polymer coated but conductive only when stressed. This isexemplified by nickel/polymer compositions of nickel content lower thanfor (iii), low enough for physical properties of the polymer to bediscernible, and high enough that during mixing the nickel particles andliquid form polymer become resolved into granules rather than forming abulk phase. This is preferred for (b) and may be unnecessary for (a) and(c). An alternative would be to use particles made by comminutingmaterial as in (v) below. Unlike (i) to

(iii), material (iv) can afford a response to stress within eachindividual granule as well as between granules, but ground material (v)is less sensitive. In making the element, material (iv) can be appliedin aqueous suspension;

(v) embedded in bulk phase polymer. This relates to (a) and (c) only.There is response to stress within the bulk phase as well as betweeninterstice walls if present.

The strongly conductive material may be for example one or more oftitanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminium,silicone, tin, chromium, molybdenum, tungsten, lead, manganese,beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium,rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper,zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metalsof the lanthanide and actinide series and if appropriate, at least oneelectroconductive agent. It can be on a carrier core of powder, grains,fibres or other shaped forms. The oxides can be mixtures comprisingsintered powders of an oxycompound. The alloy may be conventional or forexample titanium boride.

For (a) or (c) co-pending application A discloses and claims acomposition which is elastically deformable from a quiescent state andcomprises at least one electrically conductive filler mixed with anon-conductive elastomer, characterised in that the volumetric ratio offiller to elastomer is at least 1:1, the filler being mixed with theelastomer in a controlled manner, in a mixing regime avoidingdestructive shear forces, whereby the filler is dispersed within andencapsulated by the elastomer and may remain structurally intact, thenature and concentration of the filler being such that the electricalresistivity of the composition is variable in response to compression orextension forces and decreases from a given value in the quiescent statetowards a value substantially equal to that of the conductor bridges ofthe filler when subjected to either compression or extension forces, thecomposition further comprising a modifier which, on release of saidforces, accelerates the elastic return of the composition to itsquiescent state.

For (iii) and (b) a preferred composition, disclosed and claimed inco-pending application B, is an electrical conductor composite providingconduction when subjected to mechanical stress or electric charge butelectrically insulating when quiescent comprising a granular compositioneach granule of which comprises at least one substantiallynon-conductive polymer and at least one electrically conductive fillerand is electrically insulating when quiescent but conductive whensubjected to mechanical stress or electric charge.

In naked conductor or in either such composition preferably the fillerparticles comprise metal having a spiky and/or dendritic surface textureand/or a filamentary structure. Preferably the conductive fillercomprises carbonyl-derived metallic nickel. Preferred filler particleshave a 3-dimensional chain-like network of spiky beads, the chains beingon average 2.5 to 3.5 microns in cross section and possibly more than15-20 microns in length. The polymer is preferably an elastomer,especially a silicone rubber, preferably comprising a recovery-enhancingmodifier filler.

These and further details of the compositions are disclosed in the abovecited co-pending applications. If conductive ingredients of form (iii)or (iv) are used, the granules thereof are preferably of a spiky and/orirregular and/or dendritic shape.

The invention provides methods of incorporating the conductive materialinto the element. Strongly or weakly conductive particles, especially ofthe preferred shapes may be put onto or into the interstices of foams orcloths and kept in place by bonding or mechanical or frictionalconstraint, e.g. with over-large particles in slightly smallerinterstices. This can be done by simply mechanically compressing themin, or by suspending them in fluid, which is then passed through thefoam or cloth. The foam or cloth may be further processed to make itshrink and provide a better grip of the particles. Other ways to ensurethe granules remain in the element include bonding or coating film orsheet to one or more of its faces to provide a seal. If the film orsheet is electrically conductive, it also provides a means of ohmicconnection.

In the shrinking method, the element base material containinginterstices can be shrunk by using adhesives and applying pressure untilset. Another means of shrinking the base material is to heat it andapply pressure. Many heat-formable foams and cloths have been foundsuitable for this type of treatment. The area to which the pressure isapplied can be monitored for changes in electrical resistance to ensurea consistent product. As well as the amount of shrinkage, the type,size, amount and morphology of the particles used and the intersticesize also have an effect on the pressure sensitivity and resistancerange of the variable resistor. Dielectric layers can also be built inusing the arrangement of a conductive stratum above a non-conductivestratum to produce a variable resistor with an inherent dielectriclayer.

It has also been found that granules made with a non-elastomericcoating, e.g. an epoxy resin, will work in the element. It appears thatthe elastomeric nature of the base structure is sufficient for theinvention to work, though the sensitivity to pressure is usually reducedand the electrical properties of the epoxy coated granules are differentfrom those of silicone coated granules.

Whereas compression may be conveniently applied normal to the plane of asheet-like element, such element can also display electrical conductanceacross its surface, e.g. on the side of a graded structure carryingconductive polymer composition, and this conductivity may be influencedby pressure if a pressure-sensitive conductive polymer, powder orgranule is used. The other side of such a structure will display thenormal high electrical resistance unless loaded with a conductive orsemi-conductive filler during manufacture.

In such a variable resistor arranged as a pressure sensitive bridgeacross two or more ohmic conductors lying in the same plane, an increasein sensitivity may be afforded by coating the exposed back of theelement with a fully conductive layer such as metallic foil or coating.This will promote the formation of a shorter conductive path through theelement rather than across.

In a preferred variable resistor an externally connectable electrode isplaced just touching the surface of the element and a correspondingelectrode is placed opposite on the surface of the layer. In the absenceof pressure on the electrodes, the element is in a quiescent state andis non-conductive. If pressure is applied to the electrodes, the elementwill conduct when forced through the interstices of the layer.Conduction will stop when pressure is removed and the element returns toits quiescent state.

In either such arrangement, if a pressure-sensitive conductive polymer,powder or granule is used, the resistance will decrease as the pressureincreases.

In a second aspect the invention relates to electrically conductivepathways in or on conductive polymer compositions to allow electricalconnectivity to, from and between areas or points thereon. Suchcompositions and forms thereof, the subject of the above cited patentapplications and of other aspects of the invention, alter theirelectrical resistance when a load is applied. On an inflexible backingsuch as rigid metal or plastic the applied load effects mechanicalmovement of the polymer composition limited by the relative inflexiblyof the backing. However, on a flexible backing such as flexible plastic,fibrous material or foam, mechanical action on the coating will befurther modified by the mechanical response of the backing.

The invention in this aspect uses this effect in systems such as otheraspects of the invention and, in general, to provide connective pathsallowing changes of resistance to be monitored away from the point ofapplication of the actuating force. It has been found that a convenientmethod to produce conductive or semi-conductive paths on or within thesheets and structures is by applying and maintaining a stress along theroute of the required conductive path.

According to the invention in this second aspect an electrical componentcomprises a body of a material capable of increasing its electricalconductance when stressed, said body characterised by at least onelocalised region pre-stressed to permanent conductance and adapted forexternal electrical connection.

A number of ways have been found to do this:

1. To conductive polymer composition in its final shape or form butbefore it is cross-linked, stress can be applied to the area of therequired pathway during the cross-linking process. Such stress can bemechanical or electrical, directly applied or induced and can includepressure, heat, electromagnetism and other sources of radiation. Some ofthese stresses may themselves induce cross-linking along the requiredconductive path but some polymers will require a separate cross-linkingoperation to be carried out at the same time or after the formation ofthe conductive path.

2. After production and cross-linking, a permanent stress can be createdalong the required conductive path. This can be done by causing the pathto shrink using a focussed source of radiation. This can be followed bymechanical compression of the irradiated pathways to consolidate theconductive content and improve the final conductance of the path.

3. Laying polymer or adhesive, which shrinks as it cross-links or dries,on top of or within the conductive polymer composition or structure,would make the underlying. polymer composition conductive.

4. In sheets of conductive polymer composition and materials coated withconductive polymer composition a line of stitching can apply sufficientforce within and between the stitches to create a conductive path. Thinplastic foams coated with conductive granules are particularly goodmaterials for this form of the invention and flexible, touch-sensitivecircuits can be produced by this method. The thread used for thestitching can be of a standard non-conductive type and the size andtension of the stitch has an effect on the final resistance of the path.Threads containing conductive material can be used if paths with verylow resistance are required. Sheets can be produced with conductivetracks with an open-cell foam or other dielectric to keep the sheetsapart until an actuating pressure is applied to bring the sheets intomutual conduction.

The invention in its third aspect relates to polymeric sensing materialsand in particular to a sensor based on the stress-sensitive electricallyconductive polymer compositions such as those detailed in the abovecited prior patent applications.

Surprisingly it has been found that the above mentioned polymercompositions, modified polymers and structures, change electricalproperty by interaction with chemical, biological species, nuclear andelectromagnetic fields. The change in electrical property is reversibleand may give a measure of concentration of radiation flux.

According to the invention a sensor for chemical species or biologicalspecies or radiation comprises:

a) a contacting head presenting a polymer composition comprising atleast one substantially non-conductive polymer and at least one electricconductive filler and being electrically insulating when quiescent butconductive when subjected to mechanical stress or electrostatic charge;

b) means for access of a test specimen to the head;

c) means to connect the head into an electrical circuit effective tomeasure an electrical property of the polymer composition.

It is noted that in the polymer composition the encapsulant phase ishighly negative on the triboelectric series, does not readily storeelectrons on its surface and is permeable to a range of gases and othermobile molecules into the head and/or onto its surface, thus changingthe electrical property of the polymer composition.

In the contacting head the polymer composition may be for example in anyof forms (a) to (c) above.

The contacting head may include stressing means, for example mechanicalcompressing or stretching or a source of electric or magnetic field, tobring the polymer composition to the level of conductance. appropriateto the required sensitivity of the sensor.

The sensor may afford static or dynamic contacting. For staticcontacting it may be a portable unit usable by dipping the head into thespecimen in a container. For dynamic conducting, it may be supported ina flowing current of specimen or may include its own feed and/ordischarge channels and possibly pump means for feeding and orwithdrawing specimen. Such pump means is suitably peristaltic as, forexample in medical testing.

In one example the properties of the system change in real time. Thatis, under the influence of a non-uniform electric field the particlesexperience an electrophoretic force which changes the electricalproperty of the polymer structure.

In a preferred sensor the polymer composition is excited by a linear ornon-linear AC field. A range of techniques may be used to distinguishthe signal of interest from noise and from interfering signals, forexample—reactance, inductance, signal profile, phase profile, frequency,spatial and temporal coherence.

In another example the polymer composition is held in a transient stateby application of an electric charge; then increased ionisation as aconsequence of exposure to nuclear radiation changes the electricalresistivity, reactance, impedance or other electrical property of thesystem.

In a further example a complexing ionophore or other lock and key oradsorbing material is incorporated within the polymer composition. Suchmaterials include crown ethers, zeolites, solid and liquid ionexchangers, biological antibodies and their analogues or other analogousmaterials. When excited by a DC, linear AC or non-linear AC field, suchmaterials change their electrical property in accordance with theadsorption of materials or contact with sources of radiation. Suchmaterials offer the potential to narrow the bandwidth for adsorbedspecies and selectivity of the system. In a yet further example anelectride, that is a material in which the electron is the sole anion, atypical example of which might be caesium-5-crown-5 prepared byvaporising caesium metal over 15-crown-5, is incorporated within thepolymer composition. Other ionophore, zeolite and ion exchange materialsmight be similarly employed. Such a composition has a low electron workfunction, typically <<1 electron-volt, such that low DC or non-uniformAC voltages switch it from insulative to conductive phase withdecreasing time constant and increasing the bandwidth for adsorbedspecies and of the system. Such materials may be used to detect thepresence of adsorbed materials and or radiation sources.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred forms of the invention are described more fully with referenceto the accompanying drawings, in which:

FIG. 1 is an exploded view of a variable resistor shaving a flexible orrigid external connecting means;

FIG. 2 shows three variants of the element shown in FIG. 1;

FIG. 3 shows two variable resistors having a configuration of elementand external connections different from those of FIGS. 1 and 2; theseoptionally use connectors according a second aspect of the invention;and

FIG. 4 shows exploded views of two multi-function variable resistors.

Any of the variable resistors shown in the drawings may form the basisof a sensor according to a third aspect of the invention.

DETAILED DESCRIPTION OF THE INVENTION EXAMPLE

An example of a conductive foam structure for the element is as follows:a polyether open-cell foam sheet 2 mm thick and 80 ppi (32 pores per cm)cell size, is loaded with nickel/silicone coated granules in the sizerange 75-152 microns. The granules were prepared by coating INCO nickelpowder type 287 with ALFAS INDUSTRIES RTV silicone type A2000 in theproportions 8/1 by weight using rotary ablation. The granules weresieved to size and rubbed into the foam until they appeared on theunderside of the foam which is an indication of correct filling. Thefoam held 75 mg of granules per cm², corresponding to 1875 mg/cm³ onaverage through the foam after compression and about 2500 mg/cm³ in thefully loaded stratum constituting the element. The foam containing thegranules was compressed between metal sheets and heated in an oven at120C for 30 min. This process produced a very pliable pressure sensitivestructure 0.4 mm thick, which has a resistance range of more than 10¹²ohms across the thickness and which could be proportionally controlleddown to less than one ohm using only finger pressure.

Referring to the figures generally:

the words ‘upper’ and ‘lower’ relate only to positioning on thedrawings, without limitation to disposition when in use;

the circular shape of the components is illustrative only and othershapes will be chosen to suit intended use; for example a rectangularshape would be appropriate for a contacting head in the third aspect ofthe invention to provide a path for circulation of a fluid testspecimen.

Referring to FIG. 1, the variable resistor comprises external connectionmeans comprising electrodes 10 from which extend external connectors notshown. Electrodes 10 are bridged by element 14 consisting ofnickel/silicone-carrying foam as described in the Example above. Lowerelectrode 10 is supported on solid base 16. Upper electrode 10 ismovable downwards to compress element 14, under the action of means 18indicated generally by arrows and capable of action over part or all ofthe area of electrode 10. It would of course be possible to apply means18 also to the lower electrode. Electrode 10 may be a distinct membermade of hard material such as metallic copper or platinum-coated brass:in that event the action over part of the electrode area may be forexample by sloping the application of means 18 to electrode 10, or byusing an element 14 of graded thickness. Alternatively electrode 10 maybe flexible, for example metal foil, metal-coated cloth, organicallyconductive polymer, or, in a preferred switch, a coherent coating ofconductive metal on the upper and/or lower surface of element 14. Such acoating may be provided by application of metal-rich paint such assilver paint. In this variable resistor, element 14 may structurally bebased on any other material having appropriate interstices, for exampleon a thick-weave polyester cloth such as cavalry twill or on worsted.

Referring to FIG. 2, the general construction of the variable resistoris the same as in FIG. 1, but three variants 2 a-2 c of the element arepresented.

In variant 2 a the element, numbered 22, carries carbon throughout itsvolume 22+24 and nickel/silicone granules only in central region 24.When the switch is quiescent, with no stress applied by means 18, itpermits the passage of a small current by the weak conductance of thecarbon, thus providing a ‘start-resistance’ or ‘start-conductance’. Whenstress is applied by means 18, the strong conductance of thenickel/silicone composition comes into play, to an extent depending onthe area over which such stress is applied, as well as on the extent ofcompression of the composition if it has this property.

Variants 2 b and 2 c show combinations of the element with a matchinglayer of non-conductive or weakly conductive material.

In variant 2 b the element, numbered 34, is provided by thenickel/silicone-carrying upper part of a block of foam or textile, thelower part being a non-conductive or (e.g. as in 2 a) weakly conductivelayer. This combination is made by applying nickel/silicone as powder orliquid suspension preferentially to one side of the block. The boundarybetween the element and the layer need not be sharp.

In variant 2 c the element, numbered 34, may carry nickel/siliconeuniformly or gradedly, but the layer, numbered 38, is a distinct memberand may, in the assembled switch, be adhered or mechanically held incontact with element 34. This has the advantage over 2 b that the layermay be structurally different from the element, eg:

element layer

collapsed foam non-collapsed foam

. . . woven cloth

. . . net

collapsed cloth non-collapsed cloth

Referring to FIGS. 3a and 3 b, the element comprises a block 314 of foamcarrying nickel/silicone and having external connecting conductors 313embedded in it. The element may be brought to conductance by compressinga region between conductors 313 by downward action of shoe 316, whichmay have an oblique lower end so that its area of application to theelement depends on the extent of its downward movement. Instead or inaddition, shoe 316 may comprise a plurality of members individuallycontrollable to permit a desired aggregate area of application. In aminiaturised variable resistor shoe 316 may be a dot-matrix orpiezo-electric mechanism. The embedded conductors may be made of ohmicmaterial, or can be tracks of metal/polymer composition, for examplenickel/silicone, made permanently conductive by local compression by forexample shrinkage or stitching. If the embedded conductors are producedby localised compression, this may be effected in a relatively thinsheet of element, whereafter a further sheet of element may besandwiched about that thin sheet.

A variable resistor as in FIG. 3a, when used as a sensor according tothe third aspect of the invention, may conveniently form part of astatic system in which it is immersed in a fluid specimen, as well asbeing usable in a flow system.

The variable resistor shown in FIG. 3b is a hybrid using the mechanismsof FIG. 1 and FIG. 3a. It is more sensitive than the variable resistorof FIG. 3a. When compression is applied at 18, conduction betweenconductors 313 can take place also via electrode 10.

Referring to FIGS. 4, 4 a shows a variable resistor that is effectivelytwo FIG. 1 variable resistors back to back. The arrangement of twovariable resistance outputs from a single input is provided much morecompactly than when using conventional variable resistor components. TheFIG. 4a combination when used in a sensor may provide a test reading andblank reading side-by-side. FIG. 4b shows an arrangement in which twoseparate variable resistors each as FIG. 1 are electrically insulatedfrom each other by block 20. In 4 a and 4 b the variants in FIGS. 2 and3 may be used. Such combinations are examples of compactmulti-functional control means affording new possibilities in the designof electrical apparatus. In a simple example, the 4 b arrangement couldprovide an on/off switch and volume control operated by a single button.

What is claimed is:
 1. A variable electrical conductance compositehaving a first level of electrical conductance when quiescent and asecond level of conductance upon change of mechanical or electricalstress applied to said composite, said composite comprising acollapsible body of an insulating or weakly conductive materialcontaining interstices that contain granules of an insulating or weaklyconductive polymer containing particles of at least one stronglyconductive material selected from the group consisting of metals, alloysand reduced metal oxides, said granules having such a loading of saidstrongly conductive material that said granules themselves have a firstlevel of electrical conductance when quiescent and a second level ofconductance upon change of mechanical or electrical stress applied tosaid granules.
 2. A variable electrical conductance composite accordingto claim 1 in which the collapsible body comprises at least one materialselected from the group consisting of foam, net, gauze, mat and cloth.3. A variable electrical conductance composite according to claim 2which is the product of loading cells of an open-cell polymer foam withparticles of the strongly conductive material and collapsing the loadedfoam by a factor which is in the range 2 to 8 by volume but leaves itcapable of further compression.
 4. A variable electrical conductancecomposite according to claim 1 wherein the concentration of the stronglyconductive material in said collapsible body is graded.
 5. A variableelectrical conductance composite according to claim 1 in which thecollapsible body is weakly conductive and formed from a polymercontaining finely divided carbon.
 6. A variable electrical conductancecomposite according to claim 1 in a sheet-like configuration ofthickness 0.1 mm to 5.0 mm.
 7. A variable electrical conductancecomposite according to claim 1 in which the granules comprise theparticles mixed with a non-conductive elastomer.
 8. A variableelectrical conductance composite according to claim 7 in which thevolumetric ratio of particles to elastomer within the granules is atleast 0.1:1.
 9. A variable electrical conductance composite according toclaim 1 in which the particles have a surface texture comprising atleast one of a spiky surface texture and a dendritic surface textureand/or a filamentary.
 10. A variable electrical conductance compositeaccording to claim 9 in which the particles comprise carbonyl-derivedmetallic nickel.
 11. A variable electrical conductance compositeaccording to claim 7 in which the elastomer is a silicone rubber.
 12. Avariable electrical conductance composite according to claim 7 in whichthe ingredient volumetric ratio of particles to elastomer is at least1:1, the particles being mixed with the elastomer in a controlledmanner, in a mixing regime avoiding destructive shear forces, wherebythe particles are dispersed within and encapsulated by the elastomer andmay remain structurally intact, the nature and concentration of theparticles being such that the electrical resistivity of the granules isvariable in response to compression or extension forces and decreasesfrom a given value in the quiescent state towards a value substantiallyequal to that of the conductor bridges of the particles when subjectedto either compression or extension forces, the granules furthercomprising a modifier which, on release of said forces, accelerates theelastic return of the granules to their quiescent state.
 13. A variableelectrical conductance composite according to claim 6 including acollapsible layer of an insulating or weakly conductive materialcontaining interstices that are accessible to mobile fluid and which arefree of said particles.
 14. A variable electrical conductance compositeaccording to claim 1 wherein said interstices are accessible to mobilefluid.
 15. A variable electrical conductance composite according toclaim 1 in which the particles have a filamentary structure.
 16. Avariable resistor having a first level of electrical conductance whenquiescent and a second level of conductance upon change of mechanical orelectrical stress applied to said resistor, said resistor comprisingexternally connectable electrodes bridged by a collapsible body of aninsulating or weakly conductive material containing interstices thatcontain granules of an insulating or weakly conductive polymercontaining particles of at least one strongly conductive materialselected from the group consisting of metals, alloys and reduced metaloxides, said granules having such a loading of said strongly conductivematerial that said granules themselves have a first level of electricalconductance when quiescent and a second level of conductance upon changeof mechanical or electrical stress applied to said granules.
 17. Avariable resistor according to claim 16 including means effective toperform at least one of the following functions: a) to apply conductanceincreasing stress to the region of said collapsible body bridging saidelectrodes, b) to reverse conductance increasing stress to the region ofsaid collapsible body bridging said electrodes, and c) to act againstpre-existing conductance increasing stress; to the region of saidcollapsible body bridging said electrodes.
 18. A variable resistoraccording to claim 13 and including external connection by way of atleast one localized region of the collapsible body pre-stressed toconductance.
 19. A variable resistor according to claim 18 in which thecollapsible body is in sheet form and the pre-stressed region isprovided by a line of stitching.
 20. A variable resistor according toclaim 16 having externally connectable bridged electrodes embedded inthe collapsible body.
 21. A variable resistor according to claim 16wherein the concentration of the strongly conductive material in saidcollapsible body is graded.
 22. A plurality of variable resistorsaccording to claim 16 sandwiched together and actuated by a singlemechanical stressing means.
 23. A plurality of variable resistorsaccording to claim 22 including insulating means whereby the resistorsare electrically insulated from each other.
 24. A variable resistoraccording to claim 16 wherein said interstices are accessible to mobilefluid.
 25. A variable resistor according to claim 24 in which thecollapsible body is at least one material selected from the groupconsisting of foam, net, gauze, mat and cloth.
 26. A variable resistoraccording to claim 25 in which the collapsible body is the product ofloading cells of an open-cell polymer foam with particles of thestrongly conductive material and collapsing the loaded foam by a factorwhich is in the range 2to 8 by volume but leaves it capable of furthercompression.
 27. A chemical sensor comprising: a) a contacting headincluding a variable resistor having a first level of electricalconductance when quiescent and a second level of conductance upon changeof mechanical or electrical stress applied to said resistor, saidresistor including externally connectable electrodes bridged by acollapsible body of an insulating or weakly conductive materialcontaining interstices that are accessible to mobile fluid and containgranules of an insulating or weakly conductive polymer containingparticles of at least one strongly conductive material selected from thegroup consisting of metals, alloys and reduced metal oxides, saidgranules having such a loading of said strongly conductive material thatsaid granules themselves have a first level of electrical conductancewhen quiescent and a second level of conductance upon change ofmechanical or electrical stress applied to said granules, b) means foraccess of a mobile fluid containing the chemical to be sensed to thehead, and c) means to connect the electrodes into an electrical circuiteffective to detect a variation in conductance of said resistor.
 28. Asensor according to claim 27 in which the contacting head includesstressing means to bring the variable resistor to the level ofconductance appropriate to the required sensitivity of the sensor.